Casters are commonly mounted to transport vehicles, such as carts, trailers, trucks, or dollies, and allow for rolling movement of the transport vehicle along a ground surface. A caster may have a single wheel or multiple wheels, such as a dual wheel caster that includes first and second wheels. Dual wheel casters may be designed to rotate (or “swivel”) about a vertical axis (known as “swivel” casters), or may be fixed (known as “rigid” casters).
For conventional dual wheel swivel casters, the first and second wheels are mounted with the axle, such that the horizontal wheel axis is offset laterally from the vertical swivel axis. In other words, the first and second wheels are mounted such that the vertical swivel axis does not pass through the horizontal axis. During vehicle movement, this offset enables the first and second wheels to change direction to position themselves behind (or “trail”) the swivel axis, thereby maintaining proper self-alignment with the direction of travel of the transport vehicle.
Conventional dual wheel swivel casters enable the first and second wheels to change direction while rolling, thereby allowing an operator to turn or otherwise steer the vehicle. Conventional dual wheel swivel casters may have the first and second wheels rotatably coupled (known as being “locked together”) at all times, or may have the first and second wheels independently rotatable, allowing for relative rotation between the first and second wheels. There are benefits and drawbacks to both arrangements. For example, having the first and second wheels independently rotatable may cause wobbling (known as “shimmying”) that may be especially noticeable when the transport vehicle is traveling at a high rate of speed and/or is lightly loaded or empty. Having the first and second wheels locked together prevents this problem, but may cause another problem when the transport vehicle desires to turn or backup.
For example, having the first and second wheels locked together may result in “scrubbing.” When the transport vehicle reverses its direction of travel, the first and second wheels must swivel a full 180 degrees through their respective arcs to a new trailing position to realign with the new direction of travel of the transport vehicle. As the first and second wheel assemblies 44, 46 swivel about vertical axis A, the first and second wheel assemblies 44, 46 will be naturally inclined to rotate at differing speeds and direction from one another. The first and second wheel assemblies 44, 46 being rotationally locked together prevents this difference in speed and rotation thus preventing caster “shimmy” while moving on a linear path. The first and second wheel assemblies 44, 46 being locked together also causes scrubbing, where one or both of the wheels frictionally drag across (or “scrub”) the ground surface, such as a concrete or asphalt floor for example, thereby requiring an increase in force applied by the operator to the vehicle in order to complete the vehicle turn. This scrubbing undesirably increases stress on the swivel bearing components and the tread of the first and second wheels, and accelerates their wear.
What is needed is a dual wheel swivel caster where the first and second wheels are locked together in a first configuration and are independently rotatable in a second configuration, thereby capturing the benefits, while eliminating the problems, of both arrangements. Accordingly, there remains a need for improvements to dual wheel swivel casters to address the shortcomings described above.